Hi Viresh, thanks for looking into this. On 11/3/20 10:18 AM, Viresh Kumar wrote: > On 02-11-20, 12:01, Nicola Mazzucato wrote: >> Hi All, >> >> In this V3 posting I have replaced the new dt-binding with minor changes/ >> improvements for opp (since we are now using opp tables instead). >> The RFC still stands on how to make this info available to sw consumers. >> >> In the RFC, I am proposing a simple addition of a performance dependencies >> cpumask in CPUFreq core and an example of how drivers and consumers would >> make use of it. >> I also propose an alternative approach, which does not require changes in >> CPUFreq core, but - possibly - in all the consumers. >> >> This is to support systems where exposed cpu performance controls are more >> fine-grained that the platform's ability to scale cpus independently. > > I was talking to Vincent about what you are doing here and we got a > bit confused and so here are few questions that we had: > > - Based on my previous understanding, you don't want software > coordination of frequencies (which is done by cpufreq today), but > want the hardware to do that and so you want per-cpu cpufreq > policies. Correct. And this has been done for quite some time in some platforms. > > - What's the real benefit of hardware coordination ? Want to make sure > I fully understand that. The hardware coordination that is coming out by having per-cpu cpufreq policies is not new, and works just fine in most of the systems. The benefit of having per-cpu controls is that the firmware will take care of the performance of the entire system. It is purely a delegation to firmware for the performance optimizations. > > - Because of hardware co-ordination of otherwise co-ordinated CPUs, > few things break. Thermal and EAS are some of the examples and so > you are trying to fix them here by proving them the missing > information again. Correct. And for this I have proposed two ways. > > - One other thing that breaks with this is freq-invariance in the > scheduler, as the scheduler won't see the real frequencies the > various CPUs are running at. Most of the hardware we have today > doesn't have counters, like AMUs, not sure if all future ones based > on SCMI will have that too, so how are they gong to be fixed ? > Correct. freq-invariance without counters is trying to do its best based on the information it has available. It definitely relies on the knowledge of the v/f domains to work at its best so I think in the case of per-cpu it will follow the same approach as others being affected (EAS, thermal). > And if we even have to fix this (freq invariance), what's hardware > coordination giving us that makes all this worth it ? I suppose this is more a generic question for all the platforms running with h/w coordination, but for our case is that the f/w will take care of the performance optimizations for us :) > > Sorry about the long list :) No problem at all. Thank you for your time on this and I hope I have made bits clearer. Nicola >
